Introduction to viruses - AbsoluteAstronomy.com

A virus is a biological agent that reproduces inside the cells

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

of living hosts

Host (biology)

In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

. When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus, at an extraordinary rate. Unlike most living things, viruses do not have cells that divide; new viruses are assembled in the infected host cell. Over 2,000 species of viruses have been discovered.

A virus consists of two or three parts: gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s, made from either DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

or RNA

RNA

Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

, long molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s that carry the genetic information; a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

coat that protects the genes; and some viruses have an envelope

Viral envelope

Many viruses have viral envelopes covering their protein capsids. The envelopes typically are derived from portions of the host cell membranes , but include some viral glycoproteins. Functionally, viral envelopes are used to help viruses enter host cells...

of fat that surrounds and protects them when they are not contained within a host cell. Viruses vary in shape from the simple helical

Tobacco mosaic virus

Tobacco mosaic virus is a positive-sense single stranded RNA virus that infects plants, especially tobacco and other members of the family Solanaceae. The infection causes characteristic patterns on the leaves . TMV was the first virus to be discovered...

and icosahedral

Icosahedron

In geometry, an icosahedron is a regular polyhedron with 20 identical equilateral triangular faces, 30 edges and 12 vertices. It is one of the five Platonic solids....

to more complex

Bacteriophage

A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...

structures. Viruses are about 100 times smaller than bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, it would take 30,000 to 750,000 of them, side by side, to stretch to 1 centimetre (0.393700787401575 in).

Viruses spread in many different ways. Plant viruses are often spread from plant to plant by insects and other organism

Organism

In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s, known as vectors. Some viruses of animals are spread by blood-sucking insects. Each species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

of virus relies on a particular method. Whereas viruses such as influenza

Influenza

Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...

are spread through the air by people when they cough or sneeze, others such as norovirus, which are transmitted by the faecal-oral route

Fecal-oral route

The fecal-oral route, or alternatively, the oral-fecal route or orofecal route is a route of transmission of diseases, in which they are passed when pathogens in fecal particles from one host are introduced into the oral cavity of another potential host.There are usually intermediate steps,...

, contaminate hands, food and water. Rotavirus

Rotavirus

Rotavirus is the most common cause of severe diarrhoea among infants and young children, and is one of several viruses that cause infections often called stomach flu, despite having no relation to influenza. It is a genus of double-stranded RNA virus in the family Reoviridae. By the age of five,...

is often spread by direct contact with infected children. The human immunodeficiency virus, HIV

HIV

Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

, is one of several major viruses that are transmitted during sex. The origins of viruses is unclear: some may have evolved

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

from plasmid

Plasmid

In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...

s—pieces of DNA that can move between cells, while others may have evolved from bacteria.

Viral infections often cause disease in humans and animals, however they are usually eliminated by the immune system

Immune system

An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

, conferring lifetime immunity

Immunity (medical)

Immunity is a biological term that describes a state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. Immunity involves both specific and non-specific components. The non-specific components act either as barriers or as eliminators of wide...

to the host for that virus. Antibiotic

Antibiotic

An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s have no effect on viruses, but antiviral drug

Antiviral drug

Antiviral drugs are a class of medication used specifically for treating viral infections. Like antibiotics for bacteria, specific antivirals are used for specific viruses...

s have been developed to treat life-threatening infections. Vaccine

Vaccine

A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...

s that produce lifelong immunity can prevent some viral infections.

Discovery

In 1884 the French microbiologist

Microbiologist

A microbiologist is a scientist who works in the field of microbiology. Microbiologists study organisms called microbes. Microbes can take the form of bacteria, viruses, fungi, and protists...

Charles Chamberland

Charles Chamberland was a French microbiologist from Chilly-le-Vignoble in the department of Jura who worked with Louis Pasteur....

invented a filter, (known today as the Chamberland filter

Chamberland filter

A Chamberland filter, also known as a Pasteur-Chamberland filter, is a porcelain water filter invented by Charles Chamberland in 1884. It is similar to the Berkefeld filter in principle.-Design:...

or Chamberland-Pasteur filter), that has pores smaller than bacteria. Thus he could pass a solution containing bacteria through the filter and completely remove them from the solution. In the early 1890s the Russian biologist

Biologist

A biologist is a scientist devoted to and producing results in biology through the study of life. Typically biologists study organisms and their relationship to their environment. Biologists involved in basic research attempt to discover underlying mechanisms that govern how organisms work...

Dimitri Ivanovski

Dmitri Iosifovich Ivanovsky was a Russian botanist, one of the discoverers of filterable nature of viruses and thus one of the founders of virology....

used this filter to study what became known as the tobacco mosaic virus

Tobacco mosaic virus

. His experiments showed that extracts from the crushed leaves of infected tobacco-plants remain infectious after filtration.

At the same time several other scientists proved that, although these agents (later called viruses) were different from bacteria, they could still cause disease, and they were about a hundred times smaller than bacteria. In 1899 The Dutch microbiologist Martinus Beijerinck

Martinus Beijerinck

Martinus Willem Beijerinck was a Dutch microbiologist and botanist. Born in Amsterdam, Beijerinck studied at the Technical School of Delft, where he was awarded the degree of Chemical Engineer in 1872. He obtained his Doctor of Science degree from the University of Leiden in 1877...

observed that the agent multiplied only in dividing cells. Having failed to demonstrate its particulate nature he called it a "contagium vivum fluidum" to mean "soluble living germ". In the early 20th century the English bacteriologist Frederick Twort

Frederick Twort

Frederick William Twort was an English bacteriologist and was the original discoverer in 1915 of bacteriophages . He studied medicine at St Thomas's Hospital, London, was superintendent of the Brown Institute for Animals , and he was also professor of bacteriology at the University of London...

discovered viruses that infect bacteria, and the French-Canadian microbiologist Félix d'Herelle

Félix d'Herelle

Félix d'Herelle was a French-Canadian microbiologist, the co-discoverer of bacteriophages and experimented with the possibility of phage therapy.-Early years:...

described viruses that, when added to bacteria growing on agar

Agar

Agar or agar-agar is a gelatinous substance derived from a polysaccharide that accumulates in the cell walls of agarophyte red algae. Throughout history into modern times, agar has been chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium...

, would lead to the formation of whole areas of dead bacteria. Counting these dead areas allowed him to calculate the number of viruses in the suspension.

With the invention of the electron microscope

Electron microscope

An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...

in 1931 by the German engineers Ernst Ruska

Ernst Ruska

Ernst August Friedrich Ruska was a German physicist who won the Nobel Prize in Physics in 1986 for his work in electron optics, including the design of the first electron microscope.Ruska was born in Heidelberg...

and Max Knoll

Max Knoll

Max Knoll was a German electrical engineer.Knoll was born in Wiesbaden and studied in Munich and at the Technical University of Berlin, where he obtained his doctorate in the Institute for High Voltage Technology...

came the first images of viruses. In 1935 American biochemist

Biochemist

Biochemists are scientists who are trained in biochemistry. Typical biochemists study chemical processes and chemical transformations in living organisms. The prefix of "bio" in "biochemist" can be understood as a fusion of "biological chemist."-Role:...

and virologist Wendell Meredith Stanley

Wendell Meredith Stanley

Wendell Meredith Stanley was an American biochemist, virologist and Nobel laureate.-Biography:Stanley was born in Ridgeville, Indiana, and earned a BS in Chemistry at Earlham College in Richmond, Indiana. He then studied at the University of Illinois, gaining an MS in science in 1927 followed by...

examined the tobacco mosaic virus and found it to be mostly made from protein. A short time later, this virus was separated into protein and RNA parts. A problem for early scientists was that they did not know how to grow viruses without using live animals. The breakthrough came in 1931, when the American pathologist Ernest William Goodpasture

Ernest William Goodpasture

Dr. Ernest William Goodpasture was an American pathologist and physician. Goodpasture advanced the scientific understanding of the pathogenesis of infectious diseases, parasitism, and a variety of rickettsial and viral infections...

grew influenza

Influenza

Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...

and several other viruses in fertilised chickens' eggs. Some viruses could not be grown in chickens' eggs, but this problem was solved in 1949 when John Franklin Enders

John Franklin Enders

John Franklin Enders was an American medical scientist and Nobel laureate. Enders had been called "The Father of Modern Vaccines."-Life:...

, Thomas Huckle Weller

Thomas Huckle Weller

Thomas Huckle Weller was an American virologist. He, John Franklin Enders and Frederick Chapman Robbins were awarded a Nobel Prize in Physiology or Medicine in 1954 for showing how to cultivate poliomyelitis viruses in a test tube, using tissue from a monkey.Weller was born and grew up in Ann...

and Frederick Chapman Robbins grew polio virus in cultures of living animal cells. Over 2,000 species of virus have been discovered.

Origins

Viruses are found wherever there is life and have probably existed since living cells first evolved. The origin of viruses is unclear because they do not form fossils, so molecular techniques

Molecular biology

Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

have been the most useful means of hypothesising how they arose. However, these techniques rely on the availability of ancient viral DNA or RNA but most of the viruses that have been preserved and stored in laboratories are less than 90 years old. Molecular methods have only been successful in tracing the ancestry of viruses that evolved in the 20th century.

There are three main theories of the origins of viruses:

Regressive theory : Viruses may have once been small cells that parasitised

Parasitism

Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...

larger cells. Over time, genes not required by their parasitism were lost. The bacteria rickettsia

Rickettsia

Rickettsia is a genus of non-motile, Gram-negative, non-sporeforming, highly pleomorphic bacteria that can present as cocci , rods or thread-like . Being obligate intracellular parasites, the Rickettsia survival depends on entry, growth, and replication within the cytoplasm of eukaryotic host cells...

and chlamydia

Chlamydia (bacterium)

Chlamydia is a genus of bacteria that are obligate intracellular parasites. Chlamydia infections are the most common bacterial sexually transmitted infections in humans and are the leading cause of infectious blindness worldwide....

are living cells that, like viruses, can reproduce only inside host cells. They lend credence to this theory, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell.

Cellular origin theory : Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. The escaped DNA could have come from plasmid

Plasmid

In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...

s—pieces of DNA that can move between cells—while others may have evolved from bacteria.

Coevolution theory : Viruses may have evolved from complex molecules of protein and DNA at the same time as cells first appeared on earth and would have been dependent on cellular life for many millions of years.

Structure

A virus particle, known as a virion, consists of genes made from DNA or RNA which are surrounded by a protective coat of protein called a capsid

Capsid

A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...

. The capsid is made of many smaller, identical protein molecules which are called capsomers. The arrangement of the capsomers can either be icosahedral

Truncated icosahedron

In geometry, the truncated icosahedron is an Archimedean solid, one of thirteen convex isogonal nonprismatic solids whose faces are two or more types of regular polygons.It has 12 regular pentagonal faces, 20 regular hexagonal faces, 60 vertices and 90 edges....

(20-sided), helical

Helix

A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...

or more complex. There is an inner shell around the DNA or RNA called the nucleocapsid, which is formed by proteins. Some viruses are surrounded by a bubble of lipid

Lipid

Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...

(fat) called an envelope

Viral envelope

Size

Viruses are among the smallest infectious agents, and most of them can only be seen by electron microscopy

Electron microscope

. Most viruses cannot be seen by light microscopy (in other words, they are sub-microscopic); their sizes range from 20 to 300 nm

Nanometre

A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

. They are so small that it would take 30,000 to 750,000 of them, side by side, to stretch to one cm.

Genes

Genes are made from DNA (deoxyribonucleic acid) and, in many viruses, RNA (ribonucleic acid). The biological information contained in an organism is encoded

Genetic code

The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells....

in its DNA or RNA. Most organisms use DNA, but many viruses

RNA virus

An RNA virus is a virus that has RNA as its genetic material. This nucleic acid is usually single-stranded RNA but may be double-stranded RNA...

have RNA as their genetic material. The DNA or RNA of viruses consists of either a single strand or a double helix.

Viruses reproduce rapidly because they have only a few genes compared to humans who have 20,000–25,000. For example, influenza virus has only eight genes and rotavirus

Rotavirus

has eleven. These genes encode structural proteins that form the virus particle, or non-structural proteins, that are only found in cells infected by the virus.

All cells, and many viruses, produce proteins that are enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s called DNA polymerase

DNA polymerase

A DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....

and RNA polymerase

RNA polymerase

RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...

which make new copies of DNA and RNA. A virus's polymerase enzymes are often much more efficient at making DNA and RNA than the host cell's. However, RNA polymerase enzymes often make mistakes, and this is one of the reasons why RNA viruses often mutate to form new strains.

In some species of RNA virus, the genes are not on a continuous molecule of RNA, but are separated. The influenza virus, for example, has eight separate genes made of RNA. When two different strains of influenza virus infect the same cell, these genes can mix and produce new strains of the virus in a process called reassortment

Reassortment

Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. Several different processes contribute to reassortment, including assortment of chromosomes, and chromosomal crossover. It is particularly used when two similar viruses that are infecting...

Protein synthesis

Proteins are essential to life. Cells produce new protein molecules from amino acid

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

building blocks based on information coded in DNA. Each type of protein is a specialist that only performs one function, so if a cell needs to do something new, it must make a new protein. Viruses force the cell to make new proteins that the cell does not need, but are needed for the virus to reproduce. Protein synthesis

Protein biosynthesis

Protein biosynthesis is the process in which cells build or manufacture proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with amino acid synthesis and transcription of nuclear DNA into messenger RNA, which is then...

basically consists of two major steps: transcription

Transcription (genetics)

Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...

and translation.

Transcription is the process where information in DNA, called the genetic code

Genetic code

The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells....

, is used to produce RNA copies called messenger RNA

Messenger RNA

Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...

(mRNA). These migrate through the cell and carry the code to ribosome

Ribosome

A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....

s where it is used to make proteins. This is called translation because the protein's amino acid

Amino acid

structure is determined by the mRNA's code. Information is hence translated from the language of nucleic acids to the language of amino acids.

Some RNA genes of viruses function directly as mRNA without further modification. For this reason, these viruses are called positive-sense RNA viruses. In other RNA viruses, the RNA is a complementary copy of mRNA and these viruses rely on the cell's or their own enzyme to make mRNA. These are called negative-sense

Sense (molecular biology)

In molecular biology and genetics, sense is a concept used to compare the polarity of nucleic acid molecules, such as DNA or RNA, to other nucleic acid molecules...

RNA viruses. In viruses made from DNA, the method of mRNA production is similar to that of the cell. The species of viruses called retrovirus

Retrovirus

A retrovirus is an RNA virus that is duplicated in a host cell using the reverse transcriptase enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an integrase enzyme. The virus thereafter replicates as part of the host cell's DNA...

es behave completely differently: they have RNA, but inside the host cell a DNA copy of their RNA is made. This DNA is then incorporated into the host's, and copied into mRNA by the cell's normal pathways.

Life-cycle

When a virus infects a cell, the virus forces it to make thousands more viruses. It does this by making the cell copy the virus's DNA or RNA, making viral proteins, which all assemble to form new virus particles.

There are six basic, overlapping stages in the life cycle of viruses in living cells:

Attachment is the binding of the virus to specific molecules on the surface of the cell. This specificity restricts the virus to a very limited type of cell. For example, the human immunodeficiency virus (HIV) infects only human T cell
T cell
T cells or T lymphocytes belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity. They can be distinguished from other lymphocytes, such as B cells and natural killer cells , by the presence of a T cell receptor on the cell surface. They are...

s, because its surface protein, gp120
Gp120
Envelope glycoprotein GP120 is a glycoprotein exposed on the surface of the HIV envelope. The 120 in its name comes from its molecular weight of 120 kilodaltons...

, can only react with CD4
CD4
CD4 is a glycoprotein expressed on the surface of T helper cells, monocytes, macrophages, and dendritic cells. It was discovered in the late 1970s and was originally known as leu-3 and T4 before being named CD4 in 1984...

and other molecules on the T cell's surface. Plant viruses can only attach to plant cells and cannot infect animals. This mechanism has evolved to favour those viruses that only infect cells in which they are capable of reproducing.
Penetration follows attachment; viruses penetrate the host cell by endocytosis
Endocytosis
Endocytosis is a process by which cells absorb molecules by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane...

or by fusion with the cell.
Uncoating happens inside the cell when the viral capsid is removed and destroyed by viral enzymes or host enzymes, thereby exposing the viral nucleic acid.
Replication of virus particles is the stage where a cell uses viral messenger RNA in its protein synthesis systems to produce viral proteins. The RNA or DNA synthesis abilities of the cell produce the virus's DNA or RNA.
Assembly takes place in the cell when the newly created viral proteins and nucleic acid combine to form hundreds of new virus particles.
Release occurs when the new viruses escape or are released from the cell. Most viruses achieve this by making the cells burst, a process called lysis
Lysis
Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....

. Other viruses such as HIV are released more gently by a process called budding.

Effects on the host cell

The range of structural and biochemical effects that viruses have on the host cell is extensive. These are called cytopathic effects. Most virus infections eventually result in the death of the host cell. The causes of death include cell lysis (bursting), alterations to the cell's surface membrane and apoptosis

Apoptosis

Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...

(cell "suicide"). Often cell death is caused by cessation of its normal activity due to proteins produced by the virus, not all of which are components of the virus particle.

Some viruses cause no apparent changes to the infected cell. Cells in which the virus is latent

Virus latency

Virus latency is the ability of a pathogenic virus to lie dormant within a cell, denoted as the lysogenic part of the viral life cycle. A latent viral infection is a type of persistent viral infection which is distinguished from a chronic viral infection...

and inactive show few signs of infection and often function normally. This causes persistent infections and the virus is often dormant for many months or years. This is often the case with herpes viruses

Herpes simplex

Herpes simplex is a viral disease caused by both Herpes simplex virus type 1 and type 2 . Infection with the herpes virus is categorized into one of several distinct disorders based on the site of infection. Oral herpes, the visible symptoms of which are colloquially called cold sores or fever...

.

Viruses, such as Epstein-Barr virus

Epstein-Barr virus

The Epstein–Barr virus , also called human herpesvirus 4 , is a virus of the herpes family and is one of the most common viruses in humans. It is best known as the cause of infectious mononucleosis...

often cause cells to proliferate without causing malignancy, but viruses, such as papillomavirus

Papillomavirus

Papillomaviridae is an ancient taxonomic family of non-enveloped DNA viruses, collectively known as papillomaviruses. Several hundred species of papillomaviruses, traditionally referred to as "types", have been identified infecting all carefully inspected birds and mammals, but also a small number...

es are an established cause of cancer.
When a cell's DNA is damaged by a virus and if the cell cannot repair

DNA repair

DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...

this often triggers apoptosis. One of the results of apoptosis is destruction of the damaged DNA by the cell itself. Some viruses have mechanisms to limit apoptosis so that the host cell does not die before progeny viruses have been produced. HIV

HIV

, for example, does this.

Viruses and diseases

Common human diseases caused by viruses include the common cold

Common cold

The common cold is a viral infectious disease of the upper respiratory system, caused primarily by rhinoviruses and coronaviruses. Common symptoms include a cough, sore throat, runny nose, and fever...

, the flu

Influenza

Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...

, chickenpox

Chickenpox

Chickenpox or chicken pox is a highly contagious illness caused by primary infection with varicella zoster virus . It usually starts with vesicular skin rash mainly on the body and head rather than at the periphery and becomes itchy, raw pockmarks, which mostly heal without scarring...

and cold sores. Serious diseases such as Ebola

Ebola

Ebola virus disease is the name for the human disease which may be caused by any of the four known ebolaviruses. These four viruses are: Bundibugyo virus , Ebola virus , Sudan virus , and Taï Forest virus...

and AIDS

AIDS

Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...

are also caused by viruses. Many viruses cause little or no disease and are said to be "benign". The more harmful viruses are described as virulent

Virulence

Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...

.
Viruses cause different diseases depending on the types of cell that they infect.
Some viruses can cause life-long or chronic infections where the viruses continue to reproduce in the body despite the host's defence mechanisms. This is common in hepatitis B virus

Hepatitis B virus

Hepatitis B is an infectious illness caused by hepatitis B virus which infects the liver of hominoidea, including humans, and causes an inflammation called hepatitis. Originally known as "serum hepatitis", the disease has caused epidemics in parts of Asia and Africa, and it is endemic in China...

and hepatitis C virus

Hepatitis C virus

Hepatitis C virus is a small , enveloped, positive-sense single-stranded RNA virus of the family Flaviviridae...

infections. People chronically infected with a virus are known as carriers. They serve as important reservoirs of the virus. If there is a high proportion of carriers in a given population, a disease is said to be endemic

Endemic (epidemiology)

In epidemiology, an infection is said to be endemic in a population when that infection is maintained in the population without the need for external inputs. For example, chickenpox is endemic in the UK, but malaria is not...

.

There are many ways in which viruses spread from host to host but each species of virus uses only one or two. Many viruses that infect plants are carried by organism

Organism

s; such organisms are called vectors. Some viruses that infect animals and humans are also spread by vectors, usually blood-sucking insects. However, direct animal-to-animal, person-to-person or animal-to-person transmission is more common. Some virus infections, (norovirus and rotavirus

Rotavirus

), are spread by contaminated food and water, hands and communal objects and by intimate contact with another infected person, while others are airborne (influenza virus). Viruses such as HIV, hepatitis B and hepatitis C are often transmitted by unprotected sex or contaminated hypodermic needle

Hypodermic needle

A hypodermic needle is a hollow needle commonly used with a syringe to inject substances into the body or extract fluids from it...

s. It is important to know how each different kind of virus is spread to prevent infections and epidemics.

Diseases of plants

There are many types of plant virus

Plant virus

Plant viruses are viruses that affect plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses are pathogenic to higher plants...

, but often they only cause a loss of yield

Crop yield

In agriculture, crop yield is not only a measure of the yield of cereal per unit area of land under cultivation, yield is also the seed generation of the plant itself...

, and it is not economically viable to try to control them. Plant viruses are often spread from plant to plant by organism

Organism

s (vectors). These are normally insects, but some fungi, nematode worms

Nematode

The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

and single-celled organisms

Protozoa

Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

have been shown to be vectors. When control of plant virus infections is considered economical, (for perennial fruits for example), efforts are concentrated on killing the vectors and removing alternate hosts such as weeds. Plant viruses are harmless to humans and other animals because they can only reproduce in living plant cells.

Bacteriophages

Bacteriophages are viruses that infect bacteria. There are over 5,100 types of bacteriophages. They are important in marine ecology: as the infected bacteria burst, carbon compounds are released back into the environment, which stimulates fresh organic growth. Bacteriophages are useful in scientific research because they are harmless to humans and can be studied easily. These viruses can be a problem in industries that produce food and drugs by fermentation

Industrial fermentation

Industrial fermentation is the intentional use of fermentation by microorganisms such as bacteria and fungi to make products useful to humans. Fermented products have applications as food as well as in general industry.- Food fermentation :...

and depend on healthy bacteria. Some bacterial infections are becoming difficult to control with antibiotics, so there is a growing interest in the use of bacteriophages to treat infections in humans.

Innate immunity of animals

Animals, including humans, have many natural defences against viruses. Some are non-specific and protect against many viruses regardless of the type. This innate immunity is not improved by repeated exposure to viruses and does not retain a "memory" of the infection. The skin of animals, particularly its surface, which is made from dead cells, prevents many types of viruses from infecting the host. The acidity of the contents of the stomach kills many viruses that have been swallowed. When a virus overcomes these barriers and enters the host, other innate defences prevent the spread of infection in the body. A special hormone called interferon

Interferon

Interferons are proteins made and released by host cells in response to the presence of pathogens—such as viruses, bacteria, or parasites—or tumor cells. They allow communication between cells to trigger the protective defenses of the immune system that eradicate pathogens or tumors.IFNs belong to...

is produced by the body when viruses are present, and this stops the viruses from reproducing by killing the infected cell and its close neighbours. Inside cells, there are enzymes that destroy the RNA of viruses. This is called RNA interference

RNA interference

RNA interference is a process within living cells that moderates the activity of their genes. Historically, it was known by other names, including co-suppression, post transcriptional gene silencing , and quelling. Only after these apparently unrelated processes were fully understood did it become...

. Some blood cells engulf and destroy other virus infected cells.

Adaptive immunity of animals

Specific immunity to viruses develops over time and white blood cells called lymphocyte

Lymphocyte

A lymphocyte is a type of white blood cell in the vertebrate immune system.Under the microscope, lymphocytes can be divided into large lymphocytes and small lymphocytes. Large granular lymphocytes include natural killer cells...

s play a central role. Lymphocytes retain a "memory" of virus infections and produce many special molecules called antibodies

Antibody

An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...

. These antibodies attach to viruses and stop the virus from infecting cells. Antibodies are highly selective and attack only one type of virus. The body makes many different antibodies, especially during the initial infection, however, after the infection subsides, some antibodies remain and continue to be produced, often giving the host life-long immunity to the virus.

Plant resistance

Plants have elaborate and effective defence mechanisms against viruses. One of the most effective is the presence of so-called resistance (R) genes. Each R gene confers resistance to a particular virus by triggering localised areas of cell death around the infected cell, which can often be seen with the unaided eye as large spots. This stops the infection from spreading. RNA interference is also an effective defence in plants. When they are infected, plants often produce natural disinfectants which kill viruses, such as salicylic acid

Salicylic acid

Salicylic acid is a monohydroxybenzoic acid, a type of phenolic acid and a beta hydroxy acid. This colorless crystalline organic acid is widely used in organic synthesis and functions as a plant hormone. It is derived from the metabolism of salicin...

, nitric oxide

Nitric oxide

Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...

and reactive oxygen molecules

Reactive oxygen species

Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....

Resistance to bacteriophages

The major way bacteria defend themselves from bacteriophages is by producing enzymes which destroy foreign DNA. These enzymes, called restriction endonucleases, cut up the viral DNA that bacteriophages inject into bacterial cells.

Vaccines

Vaccination is a way of preventing diseases caused by viruses. Vaccines simulate a natural infection and its associated immune response, but do not cause the disease. Their use has resulted in a dramatic decline in illness and death caused by infections such as polio, measles

Measles

Measles, also known as rubeola or morbilli, is an infection of the respiratory system caused by a virus, specifically a paramyxovirus of the genus Morbillivirus. Morbilliviruses, like other paramyxoviruses, are enveloped, single-stranded, negative-sense RNA viruses...

, mumps

Mumps

Mumps is a viral disease of the human species, caused by the mumps virus. Before the development of vaccination and the introduction of a vaccine, it was a common childhood disease worldwide...

and rubella

Rubella

Rubella, commonly known as German measles, is a disease caused by the rubella virus. The name "rubella" is derived from the Latin, meaning little red. Rubella is also known as German measles because the disease was first described by German physicians in the mid-eighteenth century. This disease is...

. Vaccines are available to prevent over thirteen viral infections of humans and more are used to prevent viral infections of animals. Vaccines may consist of either live or killed viruses. Live vaccines contain weakened forms of the virus, but these vaccines can be dangerous when given to people with weak immunity

Immunodeficiency

Immunodeficiency is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. Immunodeficiency may also decrease cancer immunosurveillance. Most cases of immunodeficiency are acquired but some people are born with defects in their immune system,...

. In these people, the weakened virus can cause the original disease. Biotechnology and genetic engineering techniques are used to produce "designer" vaccines that only have the capsid proteins of the virus. Hepatitis B vaccine is an example of this type of vaccine. These vaccines are safer because they can never cause the disease.

Antiviral drugs

Over the past 20 years, the development of antiviral drug

Antiviral drug

Antiviral drugs are a class of medication used specifically for treating viral infections. Like antibiotics for bacteria, specific antivirals are used for specific viruses...

s has increased rapidly, mainly driven by the AIDS pandemic

Pandemic

A pandemic is an epidemic of infectious disease that is spreading through human populations across a large region; for instance multiple continents, or even worldwide. A widespread endemic disease that is stable in terms of how many people are getting sick from it is not a pandemic...

. Antiviral drugs are often nucleoside analogues, which are molecules very similar, but not identical to DNA building blocks. When the replication of virus DNA begins, some of these fake building blocks are incorporated. As soon as that happens, replication stops prematurely—the fake building blocks lack the essential features that allow the addition of further building blocks. Thus, DNA production is halted, and the virus can no longer reproduce. Examples of nucleoside analogues are aciclovir

Aciclovir

Aciclovir or acyclovir , chemical name acycloguanosine, abbreviated as ACV,is a guanosine analogue antiviral drug, marketed under trade names such as Cyclovir, Herpex, Acivir, Acivirax, Zovirax, and Zovir...

for herpes virus

Herpesviridae

The Herpesviridae are a large family of DNA viruses that cause diseases in animals, including humans. The members of this family are also known as herpesviruses. The family name is derived from the Greek word herpein , referring to the latent, recurring infections typical of this group of viruses...

infections and lamivudine

Lamivudine

Lamivudine is a potent nucleoside analog reverse transcriptase inhibitor .It is marketed by GlaxoSmithKline with the brand names Zeffix, Heptovir, Epivir, and Epivir-HBV.Lamivudine has been used for treatment of chronic hepatitis B at a lower dose than for treatment of HIV...

for HIV and hepatitis B virus

Hepatitis B virus

infections. Aciclovir is one of the oldest and most frequently prescribed antiviral drugs.

Other antiviral drugs target different stages of the viral life cycle. HIV is dependent on an enzyme called the HIV-1 protease

HIV-1 protease

HIV-1 protease is a retroviral aspartyl protease that is essential for the life-cycle of HIV, the retrovirus that causes AIDS.HIV PR cleaves newly synthesized polyproteins at the appropriate places to create the mature protein components of an infectious HIV virion...

for the virus to become infectious. There is a class of drugs called protease inhibitors, which bind to this enzyme and stop it from functioning.

Hepatitis C is caused by an RNA virus. In 80% of people infected, the disease becomes chronic, and they remain infectious for the rest of their lives unless they are treated. There is an effective treatment that uses the nucleoside analogue drug ribavirin

Ribavirin

Ribavirin is an anti-viral drug indicated for severe RSV infection , hepatitis C infection and other viral infections. Ribavirin is a prodrug, which when metabolised resembles purine RNA nucleotides...

combined with interferon. Treatments for chronic carrier

Asymptomatic carrier

An asymptomatic carrier is a person or other organism that has contracted an infectious disease, but who displays no symptoms. Although unaffected by the disease themselves, carriers can transmit it to others...

s of the hepatitis B virus by a similar strategy using lamivudine

Lamivudine

and other anti-viral drugs have been developed. In both diseases, the drugs stop the virus from reproducing and the interferon kills any remaining infected cells.

HIV infections are usually treated with a combination of antiviral drugs, each targeting a different stage in the virus's life-cycle. There are drugs that prevent the virus from attaching to cells, others that are nucleoside analogues and some poison the virus's enzymes that it needs to reproduce. The success of these drugs is proof of the importance of knowing how viruses reproduce.

Role in ecology

Viruses are the most abundant biological entity in aquatic environments—there are about one million of them in a teaspoon of seawater—and they are essential to the regulation of saltwater and freshwater ecosystems. Most of these viruses are bacteriophages, which are harmless to plants and animals. They infect and destroy the bacteria in aquatic microbial communities and this is the most important mechanism of recycling carbon in the marine environment. The organic molecules released from the bacterial cells by the viruses stimulates fresh bacterial and algal growth.

Microorganisms constitute more than 90% of the biomass in the sea. It is estimated that viruses kill approximately 20% of this biomass each day and that there are fifteen times as many viruses in the oceans as there are bacteria and archaea. Viruses are mainly responsible for the rapid destruction of harmful algal bloom

Algal bloom

An algal bloom is a rapid increase or accumulation in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration...

s, which often kill other marine life.
The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.

Their effects are far-reaching; by increasing the amount of respiration in the oceans, viruses are indirectly responsible for reducing the amount of carbon dioxide in the atmosphere by approximately 3 gigatonnes of carbon per year.

Marine mammals are also susceptible to viral infections. In 1988 and 2002 thousands of harbour seals were killed in Europe by phocine distemper virus

Phocine distemper virus

Phocine distemper virus is a paramyxovirus of the genus morbillivirus that is pathogenic for pinniped species, particularly seals. Clinical signs include laboured breathing, fever and nervous symptoms....

. Many other viruses, including caliciviruses, herpesviruses, adenoviruses and parvoviruses, circulate in marine mammal populations.

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